The extension have admin options where you can select SKU which will be used as gift and options.
Admin options: Enabled/Disabled extension. Force gift product qty - If this options selected to "Yes" then gift product will remain always with qty 1. Set gift product price to 0 - If this options selected to "Yes" then gift product will have zero price.
Settings & rules for the extension: The gift product can be removed from the cart and original product can be bought without it. The gift product cannot be used as usual product in the catalog. Because if it will be in the cart with any other product it will follow extension's logic. If qty of product not forced then qty in the cart will be increased. Extension may need integration if any other gift or checkout extension exist.
Features: You can easily specify sku of product which is gift. Optional ability to have 1 gift product per purchase. Automatically adjust gift product's price to 0.